Atomic Structure and Radioactivity
The nuclear model
A tiny positive nucleus (protons + neutrons) with electrons orbiting in energy levels. The atom is mostly empty space (~10⁻¹⁰ m across; nucleus ~10⁻¹⁴ m).
Isotopes
Atoms of the same element (same protons) with different numbers of neutrons. Some are unstable and decay, emitting radiation.
Types of nuclear radiation
| Type | What it is | Ionising | Penetration | Stopped by |
|---|---|---|---|---|
| Alpha (α) | 2 protons + 2 neutrons | High | Low | Paper / skin |
| Beta (β) | Fast electron | Medium | Medium | ~3 mm aluminium |
| Gamma (γ) | EM wave | Low | High | Thick lead / concrete |
Half-life
The time for half the undecayed nuclei to decay (or for the activity to halve). It's constant for a given isotope.
100% → 50% → 25% → 12.5% → 6.25% …After 3 half-lives, ⅛ (12.5%) of the original remains.
Worked example
A sample has a half-life of 5 years. What fraction remains after 15 years?
15 ÷ 5 = 3 half-lives → (½)³ = 1/8 remainsExam tip
Alpha is the most ionising but least penetrating (an inverse pattern). Count half-lives, then halve repeatedly.